"A prison within a prison": this is the common description of solitary confinement, and it conveys the extreme form of exclusion and isolation individuals suffer when kept alone, in darkness, in a small cell (roughly the size of a king size bed) for up to 23 hours a day.

Robert King, a former prisoner held in solitary confinement for 29 years in Louisiana's Angola state prison, described how his experience permanently distorted his eyesight and physical orientation. "My geography is way off. I get lost sometimes in my own neighborhood. I believe that this is a result of my solitary confinement," said King, who participated in a 14 February symposium[3] on the topic.

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The use of large-scale solitary confinement became a common feature with the rise of the modern penitentiary during the first half of the 19th century. These prisons operated under the stated belief that criminals could be rehabilitated through isolation. Solitary confinement became increasingly used during the 1970s and 1980s in the United States, spawning prison overcrowding. Along the way, solitary confinement became a tool employed to deal with mentally ill prisoners.

It's currently estimated that tens of thousands of prisoners are held in solitary confinement in the United States, according to organizations like the Center for Constitutional Rights[5]. Exact numbers are challenging pin down because the number of individuals held in solitary often go unreported and vary state by state. It's even more difficult to determine how many prisoners are held in solitary worldwide, as there is no universal definition of what constitutes solitary confinement.

The psychological effects of solitary confinement have been well documented by Craig Haney, a professor psychology at the University of California Santa Cruz. Haney has spent the past 30 inspecting numerous solitary confinement units across the United States.

He has documented cases of extreme paranoia, self-mutilation, hypersensitivity to sound, light and touch, and severe cognition dysfunction among prisoners.

"You find that prisoners begin to develop identity disorders when they have spent long periods of time without social interaction or touch," said Haney.

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"So much of who we are depends on our contact with other people, the social context in which we function, and when you remove people from that context, they begin to lose their very sense of self," Haney continued.

Assessing the neurobiological effects of solitary confinement is challenging for numerous reasons. Very few studies have been done, and working with prisons to get access to individuals in solitary confinement is difficult, said Huda Akil, a neuroscientist at the University of Michigan.

"We really don't have direct neurobiological studies of people who have gone through the experiences like those of Robert [King]," said Akil.

Scientists do not know what the brains of people who have spent time in solitary confinement look like, but there is a vast amount of knowledge about how the brain responds to elements of solitary confinement: the lack of physical interaction with the natural world, the lack of social interaction, and the lack of touch and visual stimulation.

"Each one each one of these elements by itself is sufficient to dramatically change the brain," Akil said.

Stress alters the very fabric or structure of the brain and can change how neurons communicate with each other. The brains of individuals who have experienced extreme stress are literally shriveled, Akil said. One of the shrunken areas is the hippocampus, the region of the brain that is involved in memory, spatial orientation and the control of emotions.

Touch and social interaction activate molecules in the brain called growth factors - the fertilizer that boosts brain structure and helps cells communicate with it each other.

"To me, the separation of the mental and physical is highly artificial, because there are definitely physical consequences of these experiences," said Akil.